Thin film solar cell strings

ABSTRACT

Thin film PV cells and strings of such cells that may be electrically joined with electrical conductors or electroconductive patterns are disclosed. The electrical conductors wrap or fold around the PV cells to form an electrical series connection among those cells. The electrical conductors may be formed or deposited on an electrically insulating sheet, which is then wrapped or folded around those cells. By constructing the electrical conductor and positioning the cells appropriately, an electrical connection is formed between one polarity of a given cell and the opposite polarity of the adjacent cell when the sheet is folded over. One or more dielectric materials may be applied or attached to exposed edges of the cells or conductive traces prior to folding the electrical conductors and/or electrically insulating sheet to prevent shorts or failure points.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional PatentApplication Ser. No. 61/284,924, filed Dec. 28, 2009, Ser. No.61/284,958 filed Dec. 28, 2009 and Ser. No. 61/284,956 filed Dec. 28,2009 all of which are incorporated herein by reference. Alsoincorporated by reference in their entireties are the following patentsand patent applications: U.S. Pat. No. 7,194,197, U.S. Pat. No.6,690,041, Ser. No. 12/364,440 filed Feb. 2, 2009, Ser. No. 12/424,497filed Apr. 15, 2009 and Ser. No. 12/587,111 filed Sep. 30, 2009.

BACKGROUND

The field of photovoltaics generally relates to multi-layer materialsthat convert sunlight directly into DC electrical power. The basicmechanism for this conversion is the photovoltaic effect, first observedby Antoine-César Becquerel in 1839, and first correctly described byEinstein in a seminal 1905 scientific paper for which he was awarded aNobel Prize for physics. In the United States, photovoltaic (PV) devicesare popularly known as solar cells or PV cells. Solar cells aretypically configured as a cooperating sandwich of p-type and n-typesemiconductors, in which the n-type semiconductor material (on one“side” of the sandwich) exhibits an excess of electrons, and the p-typesemiconductor material (on the other “side” of the sandwich) exhibits anexcess of holes, each of which signifies the absence of an electron.Near the p-n junction between the two materials, valence electrons fromthe n-type layer move into neighboring holes in the p-type layer,creating a small electrical imbalance inside the solar cell. Thisresults in an electric field in the vicinity of the metallurgicaljunction that forms the electronic p-n junction.

When an incident photon excites an electron in the cell into theconduction band, the excited electron becomes unbound from the atoms ofthe semiconductor, creating a free electron/hole pair. Because, asdescribed above, the p-n junction creates an electric field in thevicinity of the junction, electron/hole pairs created in this mannernear the junction tend to separate and move away from junction, with theelectron moving toward the electrode on the n-type side, and the holemoving toward the electrode on the p-type side of the junction. Thiscreates an overall charge imbalance in the cell, so that if an externalconductive path is provided between the two sides of the cell, electronswill move from the n-type side back to the p-type side along theexternal path, creating an electric current. In practice, electrons maybe collected from at or near the surface of the n-type side by aconducting grid that covers a portion of the surface, while stillallowing sufficient access into the cell by incident photons.

Such a photovoltaic structure, when appropriately located electricalcontacts are included and the cell (or a series of cells) isincorporated into a closed electrical circuit, forms a working PVdevice. As a standalone device, a single conventional solar cell is notsufficient to power most applications. As a result, solar cells arecommonly arranged into PV modules, or “strings,” by connecting the frontof one cell to the back of another, thereby adding the voltages of theindividual cells together in electrical series. Typically, a significantnumber of cells are connected in series to achieve a usable voltage. Theresulting DC current then may be fed through an inverter, where it istransformed into AC current at an appropriate frequency, which is chosento match the frequency of AC current supplied by a conventional powergrid. In the United States, this frequency is 60 Hertz (Hz), and mostother countries provide AC power at either 50 Hz or 60 Hz.

One particular type of solar cell that has been developed for commercialuse is a “thin-film” PV cell. In comparison to other types of PV cells,such as crystalline silicon PV cells, thin-film PV cells require lesslight-absorbing semiconductor material to create a working cell, andthus can reduce processing costs. Thin-film based PV cells also offerreduced cost by employing previously developed deposition techniques forthe electrode layers, where similar materials are widely used in thethin-film industries for protective, decorative, and functionalcoatings. Common examples of low cost commercial thin-film productsinclude water impermeable coatings on polymer-based food packaging,decorative coatings on architectural glass, low emissivity thermalcontrol coatings on residential and commercial glass, and scratch andanti-reflective coatings on eyewear. Adopting or modifying techniquesthat have been developed in these other fields has allowed a reductionin development costs for PV cell thin-film deposition techniques.

Furthermore, thin-film cells have exhibited efficiencies approaching20%, which rivals or exceeds the efficiencies of the most efficientcrystalline cells. In particular, the semiconductor material copperindium gallium diselenide (CIGS) is stable, has low toxicity, and istruly a thin film, requiring a thickness of less than two microns in aworking PV cell. As a result, to date CIGS appears to have demonstratedthe greatest potential for high performance, low cost thin-film PVproducts, and thus for penetrating bulk power generation markets. Othersemiconductor variants for thin-film PV technology include copper indiumdiselenide, copper indium disulfide, copper indium aluminum diselenide,and cadmium telluride.

Some thin-film PV materials may be deposited either on rigid glasssubstrates, or on flexible substrates. Glass substrates are relativelyinexpensive, generally have a coefficient of thermal expansion that is arelatively close match with the CIGS or other absorber layers, and allowfor the use of vacuum deposition systems. However, when comparingtechnology options applicable during the deposition process, rigidsubstrates suffer from various shortcomings during processing, such as aneed for substantial floor space for processing equipment and materialstorage, expensive and specialized equipment for heating glass uniformlyto elevated temperatures at or near the glass annealing temperature, ahigh potential for substrate fracture with resultant yield loss, andhigher heat capacity with resultant higher electricity cost for heatingthe glass. Furthermore, rigid substrates require increased shippingcosts due to the weight and fragile nature of the glass. As a result,the use of glass substrates for the deposition of thin films may not bethe best choice for low-cost, large-volume, high-yield, commercialmanufacturing of multi-layer functional thin-film materials such asphotovoltaics.

In contrast, roll-to-roll processing of thin flexible substrates allowsfor the use of compact, less expensive vacuum systems, and ofnon-specialized equipment that already has been developed for other thinfilm industries. PV cells based on thin flexible substrate materialsalso exhibit a relatively high tolerance to rapid heating and coolingand to large thermal gradients (resulting in a low likelihood offracture or failure during processing), require comparatively lowshipping costs, and exhibit a greater ease of installation than cellsbased on rigid substrates. Additional details relating to thecomposition and manufacture of thin film PV cells of a type suitable foruse with the presently disclosed methods and apparatus may be found, forexample, in U.S. Pat. Nos. 6,310,281, 6,372,538, and 7,194,197, all toWendt et al. The complete disclosures of those patents are herebyincorporated by reference for all purposes.

As noted previously, a significant number of PV cells often areconnected in series to achieve a usable voltage, and thus a desiredpower output. Such a configuration is often called a “string” of PVcells. Due to the different properties of crystalline substrates andflexible thin film substrates, the electrical series connection betweencells may be constructed differently for a thin film cell than for acrystalline cell, and forming reliable series connections between thinfilm cells poses several challenges. For example, soldering (thetraditional technique used to connect crystalline solar cells) directlyon thin film cells exposes the PV coatings of the cells to damagingtemperatures, and the organic-based silver inks typically used to form acollection grid on thin film cells may not allow strong adherence byordinary solder materials in any case. Thus, PV cells often are joinedwith wires or conductive tabs attached to the cells with an electricallyconductive adhesive (ECA), rather than by soldering. An example ofjoining PV cells with conductive tabs is disclosed in U.S. PatentApplication Publication No. 2009/0255565 to Britt et al. The completedisclosure of that application publication is hereby incorporated byreference for all purposes.

However, even when wires or tabs are used to form inter-cellconnections, the extremely thin coatings and potential flaking along cutPV cell edges introduces opportunities for shorting (power loss)wherever a wire or tab crosses over a cell edge. Furthermore, theconductive substrate on which the PV coatings are deposited, whichtypically is a metal foil, may be easily deformed by thermo-mechanicalstress from attached wires and tabs. This stress can be transferred toweakly-adhering interfaces, which can result in delamination of thecells. In addition, adhesion between the ECA and the cell back side, orbetween the ECA and the conductive grid on the front side, can be weak,and mechanical stress may cause separation of the wires or tabs at theselocations. Also, corrosion can occur between the molybdenum or othercoating on the back side of a cell and the ECA that joins the tab to thesolar cell there. This corrosion may result in a high-resistance contactor adhesion failure, leading to power losses.

Advanced methods of joining thin film PV cells with conductive tabs orribbons may largely overcome the problems of electrical shorting anddelamination, but may require undesirably high production costs to doso. Furthermore, all such methods—no matter how robust—require that atleast some portion of the PV string be covered by a conductive tab,which blocks solar radiation from striking that portion of the stringand thus reduces the efficiency of the system. As a result, there is aneed for improved methods of interconnecting PV cells into strings, andfor improved strings of interconnected cells. Specifically, there is aneed for strings and methods of their formation that reduceinterconnection costs and reduce the fraction of each PV cell that iscovered by the interconnection mechanism, while maintaining or improvingthe ability of the cell to withstand stress.

SUMMARY

The present teachings disclose thin film PV cells and strings of suchcells that may be electrically joined with electrical conductors orelectroconductive patterns. The electrical conductors wrap or foldaround the PV cells to form an electrical series connection among thosecells. The electrical conductors may be formed or deposited on anelectrically insulating sheet, which is then wrapped or folded aroundthose cells. By constructing the electrical conductor and positioningthe cells appropriately, an electrical connection is formed between onepolarity of a given cell and the opposite polarity of the adjacent cellwhen the sheet is folded over. One or more dielectric materials may beapplied or attached to exposed edges of the cells or conductive tracesprior to folding the electrical conductors and/or electricallyinsulating sheet to prevent shorts or failure points.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of an electrically insulating sheet with electricalconductors in accordance with aspects of the present disclosure.

FIG. 2 is a top view of the electrically insulating sheet of FIG. 1shown with first and second PV cells supported on the sheet aligned withtrailing portions of the electrical conductors.

FIG. 3 is a top view of the electrically insulating sheet of FIG. 1shown with PV cells supported on the sheet and aligned with trailingportions of the electrical conductors.

FIG. 4 is a partial view of the electrically insulating sheet of FIG. 3shown with dielectric material attached to portions of the electricalconductors and cells.

FIG. 5 is a top view of the electrically insulating sheet of FIG. 1 withdashed lines to indicate where the electrically insulating sheet will befolded.

FIG. 6 is a top view of the electrically insulating sheet of FIG. 1 withportions of the electrically insulating sheet folded along the dashedlines shown in FIG. 5.

FIG. 7 is sectional view of the electrically insulating sheet of FIG. 1taken along lines 7-7 in FIG. 6.

FIG. 8 is a sectional view of the electrically insulating sheet of FIG.1 taken along lines 8-8 in FIG. 6.

FIG. 9 is a top view of six strings of PV cells prior to connection andlamination.

FIG. 10 is a flowchart depicting methods of manufacturing strings ormodules of photovoltaic cells according to aspects of the presentteachings.

DETAILED DESCRIPTION

FIG. 1 shows an electrically insulating substrate or sheet 20 for astring of photovoltaic cells. The sheet may be configured to support aplurality of PV cells. Electrically insulating sheet 20 (also may bereferred to as “electrically insulating backing” or “filler sheet”) maybe made of any suitable materials, such as thermoplastic materialsincluding olefin-based polymers or polyolefins, ethylene vinyl acetate(EVA), ionomers, and fluoropolymers. The electrically insulating sheetmay be selected based on its recyclability (separable from othercomponents of a spent PV cell module), lamination time, degradation ofmanufacturing equipment and PV cells, adhesive properties, vaporpermeability, and/or other suitable properties.

Electrically insulating sheet 20 may include electrical conductors orelectroconductive patterns 22, as shown in FIG. 1. Electrical conductors22 may include a trailing portion 24 and one or more leading portions26. The electrical conductors may be deposited on sheet 20 via printing,plating, and/or other suitable methods. Additionally, electricalconductors 22 may be made of any suitable materials. For example,low-temperature solder (such as tin-bismuth-silver) may be plated onsheet 20 to form the electrical conductors. The solder forms ametallurgical bond with front and backside contacts of the module uponlamination. Alternatively, electrical conductors 20 may be printed usinga B-stage conductive epoxy. Alternatively, nickel may be plated on sheet20 to form electrical conductors 22 and conductive epoxy or solder pastemay be added in certain areas to ensure electrical contact. Theelectrical conductors may be deposited on only a single side of sheet22.

FIGS. 2-3 show placement of thin film PV cells 28, such as a first thinfilm PV cell 30 and a second thin film PV cell 32 on sheet 20. Firstcell 30 includes a top surface 34, a bottom surface 36, and a conductinggrid 38. Similarly, second cell 32 includes a top surface 40, a bottomsurface 42, and a conducting grid 44. Top surface 34 of first cell 30 ispositioned on trailing portion 24 of electrical conductor 22 such thatthe trailing portion contacts top surface 34 and/or conducting grid 38.Top surface 40 of second cell 32 is positioned adjacent to but spacedfrom the first cell, such as on the trailing portion of an adjacentelectrical conductor such that that trailing portion contacts topsurface 40 and/or conducting grid 44. The cells may be heat-tacked on orotherwise attached to the sheet. Although the first and second cells areshown to include conducting grids, those grids may alternatively bedeposited on sheet 20 as part of electrical conductors 22.

FIG. 4 shows dielectric material 46 that may be attached to portions ofelectrical conductors 22 and cells 28 of sheet 20 to protect againstshort circuits or failure points. The dielectric material may beattached via taping, printing, coating, or other suitable methods priorto folding sheet 20. Dielectric material 46 may be any suitableshape(s), such as a linear stripe shown in FIG. 4.

FIG. 5 shows, in dashed lines at 47, where sheet 20 may be folded toconnect leading portions 26 to bottom surfaces of cells 28. Sheet 20 maybe referred to as having a base portion 46 that includes the cells andthe trailing portions of the electrical conductors, and one or morefolded portions 48 that are folded on to the base portion. Althoughsheet 20 is shown to include two folded portions, the sheet mayalternatively include any suitable number of folded portions. Forexample, sheet 20 may include a single folded portion that covers anysuitable part(s) of the base portion when folded.

FIG. 6 shows sheet 20 with folded portions in which leading portions 26contact bottom surfaces of cells 28 to form electrical seriesconnections among cells 28 resulting in a string of connected PV cells50. For example, trailing portion 24 contacts top surface 34 of firstcell 30 such that the trailing portion is electrically connected toconducting grid 38 of the first cell. Additionally, leading portion 26contacts bottom surface 42 of second cell 32 such that the leadingportion is electrically connected to conducting grid 44 of the secondcell. Electrical conductors 22 at end portions of sheet 20 are exposedat 51 to provide positive and negative contacts for the cell. The foldedportions of sheet 20 may be heat-tacked or otherwise attached to cells28.

FIGS. 7-8 show sheet 20 and leading portion 26 of electrical conductor22 wrapping or folding around second cell 32. Trailing portion 24 isadjacent top surface 34 and spaced from bottom surface 36 of first cell30, such as within a plane parallel to the bottom surface. The trailingportion includes a first part 52 that may extend longitudinally acrossthe cell and one or more second parts 54 that may extend transverselyacross the cell toward an adjacent cell (shown in FIG. 5). From thetrailing portion to the leading portion, electrical conductor 22 wrapsor folds around the second cell such that at least a substantial portionof leading portion 26 is adjacent bottom surface 42 and spaced from topsurface 40 of second cell 32 (such as within a plane parallel to the topsurface) relative to the trailing portion. Leading portion 26 may extendlongitudinally along the second cell when folded. The leading portionincludes a first part 56, a second part 57, and a third part 58. Priorto folding sheet 20, first part 56 is spaced from third part 58 bysecond part 57 (such as shown in FIG. 4). When sheet 20 is folded, firstpart 56 is in contact with third part 58, as shown in FIG. 8. This mayprovide redundant contact to prevent the folded portion of theelectrical conductor from becoming a failure point.

FIG. 9 shows a plurality of strings 50 prior to lamination as a module60. As part of the lamination, the strings are electrically connected,such as via connection ribbons. For example, a first connection ribbonmay connect the left side of the upper three strings in FIG. 9 as thepositive module connection. A second connection ribbon may connect theleft side of the lower three strings in FIG. 9 as the negative moduleconnection. Finally, a third ribbon may connect all six strings on theright side to place the upper group of three strings and lower group ofthree strings in a series connection. Strings 50 may alternatively beconnected via other suitable method(s).

A number of methods of manufacturing strings and modules of PV cells arecontemplated by the present teachings, and an illustrative method isdepicted in FIG. 10 and generally indicated at 100. While FIG. 10 showsillustrative steps of a method according to one embodiment, otherembodiments may omit, add to, and/or modify any of the steps shown inthat figure. At step 102, electroconductive pattern(s) are deposited onan electrically insulating sheet, such as via printing or plating. Atstep 104, the top surface of a first cell is positioned on anelectroconductive pattern. The top surface of the first cell may bepositioned on a trailing portion of the pattern. At step 106, the topsurface of a second cell is positioned adjacent to, but spaced from, thefirst cell. The top surface of the second cell may be positioned on atrailing portion of an adjacent electroconductive pattern. At step 108,the sheet is folded such that one or more leading portions of theelectroconductive pattern contacts the bottom surface of the second cellto form an electrical series connection between the first and secondcells.

Method 100 also may include one or more other steps. For example, atstep 110, conducting grids are deposited on the electrically insulatingsheet where the first and second cells will be positioned. Theconducting grids may be deposited via printing, plating, or othersuitable methods. At step 112, heat is applied to bond the first andsecond cells to the sheet. At step 114, dielectric material is attachedto portion(s) of the first and second cells and the pattern. At step116, heat is applied to folded portion(s) of the sheet to bond thoseportions to the first and second cells.

The various structural members disclosed herein may be constructed fromany suitable material, or combination of materials, such as metal,plastic, nylon, rubber, or any other materials with sufficientstructural strength to withstand the loads incurred during use.Materials may be selected based on their durability, flexibility,weight, and/or aesthetic qualities.

It is believed that the following claims particularly point out certaincombinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements and/or properties may be claimed through amendment of thepresent claims or presentation of new claims in this or a relatedapplication. Such amended or new claims, whether they are directed to adifferent invention or directed to the same invention, whetherdifferent, broader, narrower or equal in scope to the original claims,are also regarded as included within the subject matter of theinventions of the present disclosure.

We claim:
 1. A thin film photovoltaic module, comprising: first andsecond thin film photovoltaic cells, each cell having a top surface anda bottom surface; and an electrical conductor having a trailing portionincluding a first part extending longitudinally across the first celland a second part extending transversely toward the second cell, and aleading portion extending longitudinally along the second cell, thefirst part of the trailing portion contacting the top surface of thefirst cell, the second part of the trailing portion disposed out ofphysical contact with each cell, and the leading portion wrapping aroundinto contact with the bottom surface of the second cell to form anelectrical series connection between the first and second cells; whereinthe leading portion includes a first unfolded part, a second part thatspans a fold, and a third part folded into direct contact with the firstunfolded part to provide redundant contact to prevent the fold frombecoming a failure point.
 2. The module of claim 1, further comprisingan electrically insulating sheet on which the electrical conductor issupported.
 3. The module of claim 2, wherein the electrically insulatingsheet wraps around the second cell.
 4. The module of claim 3, whereinthe electrically insulating sheet includes one or more olefin-basedpolymers.
 5. The module of claim 3, wherein the electrically insulatingsheet includes conducting grids that contact a portion of the first andsecond cells.
 6. The module of claim 1, wherein the first cell includesa first conducting grid and the second cell includes a second conductinggrid.
 7. The module of claim 6, wherein the trailing portion iselectrically connected to the first conducting grid and the leadingportion is electrically connected to the second conducting grid.
 8. Themodule of claim 1, further comprising dielectric material disposed on aportion of the electrical conductor and the first and second cells.
 9. Astring of thin film photovoltaic cells, comprising: first and secondthin film photovoltaic cells, each cell having a top surface and abottom surface; an electrically insulating sheet configured to supportthe first and second cells; and an electrical conductor disposed on theelectrical insulating sheet and having trailing and leading portions,wherein the trailing portion includes a first part extendinglongitudinally across the first cell and a second part extendingtransversely toward the second cell while out of physical contact withboth cells, the leading portion extends longitudinally along the secondcell and includes a first unfolded part, a second part that spans a foldand a third part, and the electrically insulating sheet folds around thesecond cell such that the first part of the trailing portion iselectrically connected to the top surface of the first cell and thethird part of the leading portion is electrically connected to thebottom surface of the second cell to form an electrical seriesconnection between the first and second cells, and the third part of theleading portion is folded into direct contact with the first unfoldedpart of the leading portion to provide redundant contact to prevent thefold from becoming a failure point.
 10. The string of claim 9, whereinthe electrical insulating sheet includes conducting grids that areelectrically connected to at least a portion of the first and secondcells.
 11. The string of claim 9, wherein the first cell includes afirst conducting grid electrically connected to the trailing portion,and the second cell includes a second conducting grids electricallyconnected to the leading portion.